1. Technical Field
The present invention is directed to determining characteristics of a fracture or crack in a formation which is traversed by a borehole. More particularly, the present method is directed to a method of determining the depth of the fracture, relative to the top of the borehole, as well as the width of the fracture.
Fractures are modelled herein as two parallel planes separated a predetermined distance. Fracture width is defined as the distance of separation between the two parallel planes.
2. Background Information
Methods are known in the art for determining the relative depth and width of fractures which intersect a borehole. For example, in U.S. application Ser. No. 947,946 (now U.S. Pat. No. 4,831,600) entitled "Borehole Logging Method For Fracture Detection and Determination," by Hornby and Johnson, assigned to the same assignee as the present invention, and herein incorporated by reference, employs sonic techniques.
Specifically, a sonic logging sonde having an acoustic source and at least one acoustic detector is deployed in the borehole. The source generates a tube wave, commonly referred to as a Stoneley wave, which propagates through the borehole. Based on the travel time and energy content of the Stoneley wave produced by the acoustic source as received by the detector, the depth and/or width of a fracture which has intersected the borehole can be determined.
In Hornby et al., the acoustic detector is preferably located above the source, relative to the top of the borehole. The Stoneley wave generated by the source travel through the borehole towards the detector, where it is received as a first signal. The signal continues to travel through the borehole where it possible encounters a fracture. The fracture will interfere with the Stoneley wave, absorbing some of the Stoneley wave's energy. The fracture will also appear as a source of the remaining energy, causing the remaining energy to be reflected away from the fracture through the borehole. The energy reflected in the direction of the logging sonde will be received by the detector as a second signal.
The first and second signals are combined so as to be indicative of the depth and/or width of the fracture. In general terms, the time it takes for the second signal to reach the detector is indicative of the distance of the fracture from the detector; the strength of the second signal (where the fracture has interfered) relative to the strength of the first signal (where no fracture has interfered) is indicative of the width of the fracture.
Although the method described by Hornby et al. is an excellent method of determining the relative depth and width of fractures which intersect a borehole, the method identifies fracture width as a single value, based on the average width of the fracture over the borehole azimuth. However, as known by those skilled in the art, fracture width can vary substantially over the borehole wall.
Additionally, Hornby et al. fail to specify either the angle of the fracture, commonly referred to as "dip angle" or its azimuthal direction. As appreciated by those skilled in the art, information directed to azimuth and dip are valuable. For example, more precise production schemes, for example, in which direction to drain the well, can be determined.